1. Field of the Invention
The invention relates to a fusible link in an integrated semiconductor circuit, in which a conductor track has a cross-sectional constriction as a desired fusing point. The invention also relates to a process for producing a fusible link in an integrated circuit.
Integrated semiconductor circuits frequently have fusible links, which are used to introduce information into an already finished integrated circuit. This is carried out in such a way that the fusible link is either ignited or not ignited. In other words, a conductor track is either interrupted or not interrupted. Fusible links are used, for instance, to secure personal data in chip cards, to personalize standard components in motor vehicles, to set precise resistances or resistance ratios in analog circuits, and as a memory element in a PROM or for increasing the yield in memory components by cutting off defective memory cells.
Typically, a fusible link includes a conductor track that has a constriction of its cross section at the desired fusing point. That constricted cross section typically has a vertical extent of 0.1 to 1 .mu.m and a horizontal length of 0.5 to 3 .mu.m. The fusible link is embedded in one or more insulating layers, preferably silicon dioxide, depending on the layer sequence of the semiconductor component.
The interruption of the conductor track is carried out through the use of a current pulse that leads to melting open of the fuse and an insulation layer surrounding it. In a typical fuse made of polysilicon, approximately 20 mA at 12 V for a period of 5 to 10 .mu.sec are required to achieve complete severing. Only about 1 to 10% of this ignition energy is needed for melting open the constricted conductor track itself. In other words, by far the majority of the ignition energy is dissipated to the surrounding insulation layers. A problem of fuses that are interrupted in this way is the danger of revitalization. In other words, the original conductivity may be partly reestablished as a result of the effects of temperature and voltage. Upon ignition, an approximately spherical region with a diameter of a few micrometers (typically 2 to 3 .mu.m) melts open. That causes the destruction of the surrounding layers, and especially of the passivation layer above. As a result, contaminants such as alkali ions can penetrate more easily and can impair the reliability of the components of the integrated circuit.
Fusible links are also used as a memory element in PROM memory components. In them, a memory cell includes one fusible link and one diode. Since the performance on melt-through is very high and the attained reliability is low, such memories are alternatively made with so-called floating gate or SONOS structures. The Fowler-Nordheim tunneling effect or hot electron injection is used for programming. A disadvantage of the Fowler-Nordheim tunneling effect is the requisite high programming voltages of about 15 V. The writing times are about 10 .mu.sec to 1 msec. In programming by hot electron injection, relatively high currents of a few milliamps are necessary, and writing times are about 10 .mu.sec.